FIG. 1 shows, in this context, a circuit arrangement known from the prior art. This has an input having a first E1 and a second input terminal E2 between which a DC supply voltage, preferably the so-called DC link voltage UZW, is applied. A series connection including a first S1 and a second electronic switch S2 in half-bridge configuration and a shunt resistor RS is connected between the input terminals E1 and E2. Between the switches S1, S2, a half-bridge midpoint HBM1 is implemented which is connected to a first output terminal A1 via an inductor L1. A second half-bridge midpoint also present is not shown in FIG. 1. The first output terminal A1 forms together with a second output terminal A2 an output for coupling to at least one discharge lamp.
The switches S1, S2 are alternately rendered conducting and nonconducting in push-pull manner in the normal way by a control unit 10, in particular with a frequency ≧20 kHz. Formed between the switch S2 and the shunt resistor RS is a tapping point AP which is connected to a first input terminal EA1 of the control unit 10 via an integrator device including an ohmic resistor R3 and a capacitor C1 and is used to supply a measurement signal MS1. The input terminal EA1 is connected via a driver device 12 to a device 14 for detecting overload operation of the at least one discharge lamp. A voltage divider including the ohmic resistors R1 and R2 is connected in parallel with the shunt resistor R. The tapping point of the voltage divider R1, R2 is coupled to a second input terminal EA2 of the control unit 10 to supply a second measurement signal MS2. The measurement signal MS2 at the input terminal EA2 is fed via a driver device 16 to a device 18 for ignition control of the at least one discharge lamp.
Devices for detecting overload operation and devices for ignition control are sufficiently known from the prior art.
Here the need for ignition control results from the fact that a predefinable maximum ignition voltage must not be exceeded, in order to prevent damage to a generic circuit assembly. On the other hand, ignition control is used to disconnect the circuit assembly when the discharge lamp is removed in order to prevent malfunctions or rather avoid posing a hazard to persons who might touch the output terminals A1 and A2. For ignition control purposes, the peak value ÛS of the voltage US dropped across the shunt resistor RS is evaluated.
The need to detect overload operation results from the fact that circuit assemblies with a constant output current characteristic have the disadvantageous property of using significantly increased system power to operate discharge lamps that possess an excessively high lamp voltage because of manufacturing-related impurities. This applies in particular to compact fluorescent lamps. Without suitable countermeasures, overheating of the discharge lamp and/or circuit assembly may occur. For this purpose the output power Pout is monitored during operation of the circuit assembly. At constant DC link voltage UZW, a linear relationship exists between this power and the average value ĪS of the current IS through the shunt resistor RS, i.e. Pout=ĪS×UZW.
In the context of ignition control and overload control, the switch control device 20 is designed to vary the frequency of the control signals of the switches S1 and S2 appropriately.
The control unit 10 has a switch control device 20 which is connected to the device 14 for detecting overload operation and the ignition control device 18. The switch control device 20 is designed to modify the control signals for the first S1 and the second electronic switch S2 as a function of the output signals of the device 14 for detecting overload operation and of the ignition control device 18. The shunt resistor RS is used here for overload control parameterization and the voltage divider R1, R2 for ignition control parameterization.
The disadvantage of this known circuit assembly is the fact that, to implement the two functions—ignition control and overload control—two measurement signals must be fed to the control unit 10, namely the measurement signals MS1 and MS2 as shown in FIG. 1. Two measurement lines are required for this purpose, which means that two pins have to be provided on the housing of the control unit 10.